Spectrographs measure the light and color wavelengths that come from objects, revealing key information, such as the chemical makeup of celestial bodies and their surrounding gases.

The elaborate repair requires Massimino to remove 111 screws with a tool made specifically for the job.

Additional instruments to be installed by NASA astronauts:

 a new spectrograph more sensitive to ultraviolet color bands, which will allow Hubble to more deeply probe the large-scale structure of the universe;

 new gyroscopes and batteries that will keep the satellite powered and correctly aimed.

 a mechanism for a rocket to attach itself to Hubble and safely guide it back to Earth when the telescope is finally decommissioned.

Safety First

Hubble, which now orbits about 350 miles (550 kilometers) above Earth, has not been serviced since 2002.

Safety concerns following the destruction of the shuttle Columbia in 2003 delayed this final servicing mission, which was originally scheduled for 2004, then 2006.

During the upcoming mission, a second space shuttle—Endeavor—will sit on the launch pad, ready to blast into space and aid the seven Atlantis astronauts in the event of an emergency.

After the Columbia disaster, manned missions were required to have mid-space rescue plans. All previous post-Columbia shuttle missions have gone to the International Space Station, which is considered an adequate mid-space safe haven.

But the space station is too far from Hubble to be part of a rescue, making the second standby shuttle mission necessary.